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9ИССЛЕДОВАНИЕ ДЕТАЛЕЙ МЕХАНИЗМОВ И МАШИН ИОННО-ПЛАЗМЕННЫМ
МЕТОДОМ
Юров В.
кандидат физ.-мат. наук, доцент Карагандинский университет имени Е.А. Букетова,
Казахстан, Караганда Бердибеков А. доктор философии (PhD) Национальный университет обороны имени Первого Президента РК - Елбасы Казахстан, Нур-Султан Бельгибеков Н.
Магистр, ТОО «Research & development центр «Казахстан инжиниринг» Казахстан, Нур-Султан
INVESTIGATION OF PARTS OF MECHANISMS AND MACHINES BY THE ION-PLASMA
METHOD
Yurov V.
Candidate of phys.-mat. sciences, associate professor Karaganda University named after E.A. Buketov Kazakhstan, Karaganda Berdibekov A. Doctor of philosophy (PhD) National Defense University named after the First President of the republic of Kazakhstan - Elbasy Kazakhstan, Nur-Sultan Belgibekov N.
Master, LLP "Research & Development center "Kazakhstan engineering" Kazakhstan, Nur-Sultan
Аннотация
Хотя способы получения наноструктурных покрытий довольно разнообразны, но все они основаны на механизме интенсивной диссипации энергии, обобщенной в трех стадиях формирования. На первой стадии идет процесс зародышеобразования. Вторая стадия представляет собой формирование вокруг нано-кристаллических зародышей аморфных кластеров, которые - на третьей стадии - объединяются в межкри-сталлитную фазу с образованием диссипативной структуры. Применительно к нашим задачам, мы использовали решение задачи Стефана путем замены потока электронов на поток адатомов на поверхности подложки. Тогда для плотности потока атомов или ионов, формирующих покрытий, мы получили уравнение, включающее функцию Бесселя нулевого порядка, которое показывает, что процесс формирования ионно-плазменных покрытий согласуется с островковой моделью. В работе экспериментально исследовано влияние на структуру и физические свойства покрытий следующих параметров: давление реакционного газа в рабочей камере; потенциал основы; ток разряда дуги; свойства материала катода; температура подложки. Определены их оптимальные значения при генерации многокомпонентных потоков. Показано также, что микротвердость и модуль Юнга уменьшаются с уменьшением фрактальной размерности структуры ионно-плазменного покрытия.
Abstract
Although the methods for obtaining nanostructured coatings are quite diverse, they are all based on the mechanism of intense energy dissipation, generalized in three stages of formation. At the first stage, the nucleation process takes place. The second stage is the formation of amorphous clusters around the nanocrystalline nuclei, which, in the third stage, are combined into an intercrystalline phase with the formation of a dissipative structure. In relation to our problems, we used the solution of the Stefan problem by replacing the electron flux with the adatom flux on the substrate surface. Then, for the flux density of atoms or ions forming the coatings, we obtained an equation that includes the zero-order Bessel function, which shows that the process of formation of ion-plasma coatings is consistent with the island model. The work experimentally investigated the effect on the structure and physical properties of coatings of the following parameters: pressure of the reaction gas in the working chamber;
base potential; arc discharge current; properties of the cathode material; substrate temperature. Their optimal values for the generation of multicomponent flows have been determined. It is also shown that the microhardness and Young's modulus decrease with a decrease in the fractal dimension of the structure of the ion-plasma coating. Ключевые слова: нанокристалл, покрытие, диссипация, задача Стефана, микротвердость, фракталл. Keywords: nanocrystal, coating, dissipation, Stefan problem, microhardness, fractal.
Introduction
Among the methods of applying protective coatings based on the action of high-energy particle and quantum fluxes on the surface of a part, vacuum ionplasma methods are attracting great attention [1-4]. Their characteristic feature is the direct conversion of electrical energy into the energy of technological impact, based on structural-phase transformations in the condensate deposited on the surface or in the very surface layer of a part placed in a vacuum chamber.
The main advantage of these methods is the possibility of creating a very high level of physical and mechanical properties of materials in thin surface layers, the application of dense coatings from refractory chemical compounds, as well as diamond-like ones, which cannot be obtained by traditional methods. In addition, these methods allow you to:
- to ensure high adhesion of the coating to the substrate;
- uniformity of coating thickness over a large area;
- vary the composition of the coating in a wide range, within one technological cycle;
- to obtain a high purity of the coating surface;
- ecological cleanliness of the production cycle.
1. Formation of ion-plasma coatings
To improve the operational properties of parts of mechanisms and machines, special steels and alloys are used, the volume of world production of which is steadily declining. This is due to both their high cost and the development of new technologies that make it possible to increase the hardness, wear resistance and other properties of parts made of cheaper grades of steel. Such technologies include methods of chemical-thermal treatment of parts and the application of special coatings to them. One of the methods of chemical-thermal treatment of steel parts is the method of ion-plasma nitriding. In recent years, this method has been actively studied both theoretically and experimentally. A number of industrial installations for ion-plasma nitriding have been created in different countries. Although the methods for obtaining nanostructured materials and coatings are quite diverse [5], they are all based on the mechanism of intense energy dissipation, generalized in three stages of formation. At the first stage, the process of nucleation takes place, which, due to the lack of appropriate thermodynamic conditions, does not transform into mass crystallization. The second stage is the formation of amorphous clusters around the nanocrys-talline nuclei, which, in the third stage, are combined into an intercrystalline phase with the formation of a dissipative structure.
Each of these stages is a complex process. Suffice it to point out the process of formation of new phase embryos, the theory of which has been developing for more than 100 years and the basic provisions of which were laid down by Gibbs and then developed by Volmer, Becker and Döring, Stransky and Kaishev,
Frenkel, Zel'dovich [6].
Despite various modifications and refinements of the classical theory of nucleation and its completely correct qualitative description of the process of nucleation of crystallization centers, it is still far from the agreement of the experimental and theoretical data. This is due to the following circumstances:
- the theory is based on the rough assumption that microscopic clusters of atoms have macroscopic characteristics (in particular c) and, in general, equilibrium thermodynamic parameters;
- practically, in all works devoted to the initial stage of the emergence of a new phase (including works of recent years), the formation of a metastable phase is considered instantaneous;
- the theory (and its modifications) contains a number of parameters that do not have a strictly quantitative solution.
With regard to our problems, we modified the cluster formation model proposed in [7]. For the equilibrium number of atoms in a cluster, we obtained:
No =
1 lnn„
c n„
G kT
1/2
(1)
xx0 ^ J
Let us estimate the number of particles in a cluster for coating titanium nitride: G° ~ 410 kJ/mol; c ~ 0.001; ln(no)/no ~ 0.02; k = 1.38 10-23, T = 300 K. Then No ~ 60 titanium atoms. The value obtained by us correlates with the number of atoms N in the nucleus during the homogeneous formation of nanoclusters of various metals. In this case, the cluster size is r ~ 1 nm.
The third stage of coating formation is even more complicated, when the growth of the film is determined by the conditions at the mobile interface. Problems of this kind are called the "Stefan problem" [8]. From a mathematical point of view, boundary value problems of this type are fundamentally different from classical problems [9]. Due to the dependence of the size of the flow transfer region on time, the classical methods of separation of variables and integral Fourier transforms are inapplicable to this type of problems, since, while remaining within the framework of classical methods of mathematical physics, it is not possible to coordinate the solution of the equation with the motion of the phase interface.
Any attempts to analytically obtain an exact solution to a generalized boundary value problem in a domain with a boundary moving according to an arbitrary law led to a system of Voltaire integral equations of the second kind, which could not be solved due to the complexity of the kernels of the equation of the system [9]. An overview of modern achievements in solving the Stefan problem is given in the monograph [10].
With regard to our problems, we used the solution of the Stefan problem proposed in [11], replacing the
electron flux with the adatom flux on the substrate surface. Then, for the flux density of atoms or ions forming the coatings, we obtain the following equation:
where J0 is the zero-order Bessel function; r - current coordinate; R is the geometric size of the substrate; P(t) is the speed of the phase separation. The graph of the function p(r) is shown in Figure 1.
Figure 1 - Graph of the function p(r) for r > 0
From Figure 1 and formula (2) it follows that the flux of atoms forming the coating disintegrates, forming "islands". Our result is consistent with the island model of the formation of ion-plasma coatings [12]. It also follows from formula (2) that with an increase in the growth rate of the coating p (t), the atomic flux density decreases and this leads to a deterioration in the quality of the coating. This situation is typical when the thermophysical parameters of the substrate and the applied material are very different. To remedy this situation, it is necessary to use composite (multiphase) cathodes for coating.
NK
0
Figure 2 -
AFM image of the surface of the Fe-Al coating
2. Influence of technological parameters on the structure of ion-plasma coatings
The following parameters have a direct effect on the structure and physical properties of coatings obtained by ion-plasma deposition [1-4, 12]: pressure of the reaction gas in the working chamber; base potential;
Let's make one more remark. Taking into account the surface tension at the interface leads to a significant change in the kinetics of the formation of coatings. Figures 2 and 3 show the structure of two multiphase coatings with different surface tension.
In the first case (Figure 2) we observe open dissi-pative structures, and in the second (Figure 3) - a globular structure. We will discuss in more detail the effect of surface tension on the structure of coatings below. Within the framework of the Stefan problem, we know of only one work [13], in which the role of surface tension is taken into account.
0
Figure 3 -
AFM image of the surface of the Zn-Al coating
arc discharge current; properties of the cathode material; substrate temperature. Figures 4 and 5, as an example, show electron microscopic and AFM images of composite coatings at substrate temperatures of 350 and 450 °C.
350 °C 450 °C
Figure 4 - Electron microscopic image Zn-Al coatings
350 °C
450 °C
Figure 5 - AFM image of the Zn-Al coating
Figure 4 shows that at a temperature of 450 °C, the zinc phase coagulates into larger fragments, while the rest (most part) becomes more uniform. This is clearly seen in the AFM image (Figure 5). A similar situation is observed for other coatings.
The optimum substrate temperature for all composite coatings was found to be about 400 °C. The refinement of the grain structure of the coating material with an increase in the substrate temperature is accompanied by an increase in hardness to a certain critical average nanograin size. A decrease in hardness with a further decrease in the average grain size in the coating is due
to slippage along grain boundaries (rotational effect). In this case, to further increase the hardness, it is required to slow down the sliding process along grain boundaries. Such deceleration can be achieved through the formation of a corresponding nanostructure with strengthening of grain boundaries using multicompo-nent flows.
Figure 6 shows an electron microscopic image of a Cr-Mn-Si-Cu-Fe-Al + Ti coating in an argon atmosphere. Titanium grains with a size of 1 to 10 microns in diameter are clearly visible. Materials with such a grain size are usually called coarse-crystalline [1].
«.iv:
* ■ /r » '
v-Vv y. • a ■ ■
1 > i. -tM
' , ! Y 'I •
Figure 7 - Electron microscopic image of the Cr-Mn-Si-Cu-Fe-Al + Ti coating in a nitrogen atmosphere
Figure 6 - Electron microscopic image of the Cr-Mn-Si-Cu-Fe-Al + Ti coating in argon
The results of quantitative XPS analysis showed that the content of Mn, Si, Cu and Al is less than 1 wt. %. In a nitrogen environment, the structure of the coating changes dramatically (Figure 7) due to the formation of titanium nitride. In this case, the average grain size is (100-150) nm. Such coatings are called submicrocrystalline [1]. The results of a quantitative XPS analysis of the Cr-Mn-Si-Cu-Fe-Al + Ti coating in a nitrogen atmosphere showed that the contents of chromium, titanium, and nitrogen are close to each other. This suggests that in addition to the formation of titanium nitride, the process of formation of chromium nitride is also taking place. Figure 7 shows that micro-crystallites of titanium and chromium nitrides have a
10.2um x 10.2um x 65532.0(null) [256 x 256]
Irani jftfj
■ft vif/.
yJu I
V^ifl V« > I \y
■■■ I IMHv à
Figure 8 - The appearance of a droplet phase with an increase in current over 130 A for a composite coating Cr-Mn-Si-Cu-Fe-Al (AFMimage)
preferred orientation (presumably in the (200) direction), which is also different from the spherical symmetry of pure titanium microcrystallites (Figure 6).
An increase in the arc discharge current leads to an increase in the thickness of the coating; however, with an increase in the current above 130 A, the perfection of the structure decreases and the amount of the droplet phase sharply increases, which is the reason for a decrease in the adhesion strength of the substrate to the coating (Figure 8). At a low discharge power (arc current < 20-30 A), due to a decrease in the ionization coefficient of the plasma, neutral particles of the reaction gas and cathode are "walled in" into the film, which contributes to an increase in the concentration of coating defects (Figure 9).
Figure 9 - Increase in the concentration of defects in the Mn-Fe-Cu-Al coating at low discharge power (AFM image)
We have investigated the dependence of the properties of composite coatings on the nitrogen pressure in the working chamber; in this case, the current strength,
reference voltage, cathode material, conditions of fixing and heat removal, and the time of cleaning and spraying processes remained constant. Table 1 shows the results for microhardness.
Table .1
Dependence of coating microhardness on gas pressure in the chamber
Residual gas pressure in the chamber, mm. hg. art. Vickers microhardness, HV
Al-Fe Zn-Cu-Al Zn-Al Al-Fe
10-8 0,662 - - 0509
10-7 0,660 - - 0,512
10-6 0,600 0,573 0,569 0,514
10-5 0,610 0,600 0,520 0,470
At a nitrogen pressure P = (0.058-0.81) Pa, a fine dense texture is formed, close to the stoichiometric composition, which is characterized by the optimal, from the point of view of metallic properties, the ratio of the metallic and ionic components of the bond. In this case, the content of the droplet phase decreases, and the number of pores and delamination increases. With a further increase in pressure, a large number of free ions leads to a sharp increase in the number of pores and delamination.
After analyzing the results of the study, we can conclude that the samples obtained at a nitrogen pressure P = (0.081-0.81) Pa have the most uniformly distributed fine dense structure, the minimum content of the droplet phase, pores, sagging, exfoliation and the highest values of microhardness.
3. Fractal structures of multiphase coatings
To show that the structure under consideration is fractal, it is possible to use the ratio of the perimeter and area of planar figures in relation to the structures formed in the surface layer of the material. It is known [14] that for each family of flat geometric figures, the ratio of the perimeter L to the area S:
L
(3)
remains constant and does not depend on the size of the figure.
However, if the structure is fractal, then, as shown in [15], it should be replaced by:
y I
L(¿)
1/D
S(¿)
1/2
(4)
where D is the Hausdorff fractal dimension of the structure under consideration.
Ratio (4) does not depend on the size of the fractal structure, but depends on the choice of the standard of length I, since the length of the self-similar boundary of the fractal L(l) depends on the length of the standard with which it is measured and L(l) ^ ® as I ^ 0. The fractal area remains finite as I ^ 0 and is defined as:
S(£)=m2, (5)
where N is the number of cells with area I2 required to cover a flat fractal.
In general, fractal structures with self-similar boundaries satisfy the perimeter-area ratio [14]:
«1-D rn//)\TD/2
L(¿)=CTD [SCOf
(6)
where C is the coefficient of proportionality.
Relation (6) is observed for any standard of length I, small enough to measure the smallest of the fractal sets. We calculated fractal dimensions from AFM images at the height of the median plane (Figure 10). The lower the value of the fractal dimension Dk of the contours, the more large-scale structures are present in the material. The value of the fractal dimension of the structure determines its spatial character. Accordingly, the lower the Ds value, the more "island" the structure has.
Cr-Mn-Si-Cu-Fe-Al
Zn-Al Mn-Fe-Cu-Al
Figure 10 - Fractal structures of multiphase coatings
Al-Fe
Table 2 shows the measured values of physical quantities for coatings with a globular structure. Table 2 shows that microhardness and Young's modulus decrease with decreasing fractal dimension of the coating
structure. Note also that the fractal dimension of the coating structure is lower than the fractal dimension of the metal substrate structure.
Table 2
Values of microhardness and elastic modulus for coatings with different fractal dimensions
Composite coating Microhardness, GPa Young's modulus, GPa Fractal dimension structures Ds
Cr-Mn-Si-Cu-Fe-Al 55,0 0,6 1,89
Zn-Al 42,0 0,5 1,81
Mn-Fe-Cu-Al 36,0 0,3 1,79
Conclusion
The main results obtained in this article are as follows:
- from the experimental data given in the article, it follows that before using the cathode for ion-plasma treatment of a material, it is necessary to investigate its microstructure and make sure that this cathode is a solid solution;
- it is shown that the XPS method makes it possible to determine the composition of the coating with high accuracy, which is very important for the directed synthesis of composite coatings with desired properties;
- it has been shown that the AFM method makes it possible to build a bridge between the atomic structure of the coating and its macroscopic properties;
- the influence of the following parameters on the structure and physical properties of coatings obtained by the method of ion-plasma deposition was experimentally investigated: pressure of the reaction gas in the working chamber; base potential; arc discharge current; properties of the cathode material; substrate temperature. Their optimal values for the generation of multicomponent flows have been determined;
- it is shown that microhardness and Young's modulus decrease with decreasing fractal dimension of the coating structure. Note also that the fractal dimension of the coating structure is lower than the fractal dimension of the metal substrate structure.
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